99 Jeep Grand Cherokee 4.0L idles rough on start up?

I have a 1999 Jeep Grand Cherokee 4.0L with 180,000 miles. It runs fine other than it occasionally idles rough on start up and acts like it is going to die (the whole vehicle shakes). After a few seconds (it seems like many minutes) it smoothes down and runs fine. It does not do it all the time so I have not been able to reproduce it for my mechanic to diagnose. Anyone else had this problem?